This invention relates to a structural member for color picture tube of a color television, more particularly to a shadow mask.
The shadow mask is one of the members of the color picture tube which are liable to be inversely affected by the thermal expansion thereof due to the temperature elevation caused by electron beams shot from electron guns of the color picture tube to collide with the members, and which are required to be prepared in a higher density and minuteness by a photoetching method.
Heretofore, there has been well known in the art the so called shadow mask tube, in which a shadow mask is employed, as a picture tube for color television.
As shown in FIG. 1, which is a perspective view of a shadow mask tube using a delta type electron gun, the shadow mask tube is constituted by providing a shadow mask 3 having a number of perforations 3a, 3b, . . . for passing electron beams between the three-electron guns 1a to 1c and the tri-color fluorescent face 2. The shadow mask 3 has the function of rearranging the electron beams shot from the three electron guns 1a to 1c against a specific perforation for passing electron beams, for example, 3c, as the target to have the correct beam spots projected on the respective colors' fluorescent portions 2a to 2c of the tri-color fluorescent face 2.
The above perforations 3a, 3b, . . . for passing electron beams are generally protected against generation of scattered electrons by working a face 4 confronting the fluorescent face 2 (hereinafter referred to as "mask face") into a shape engraved in a semi-spherical shape, as shown in an enlarged sectional view in FIG. 2.
The relative positions, sizes and shapes of the electron beam-passing perforations 3a, 3b, . . . in the shadow mask 3 are set in sufficiently high precision. In this connection, if the working precision of the above perforations 3a, 3b, . . . is poor, there may be caused image deterioration by blurring of colors, color irregularities or the like which is called as doming phenomenon.
On the other hand, in these days, there is an increasing general demand of "fineness of texture" for television pictures, and the transmitting system is being changed to what is called the high quality television system, for which the scanning line number is required to be increased to twice as much as that of the conventional system. Thus, to cope with such a demand, it is strongly desired to develop a picture tube capable of reproducing pictures which are clear and of fine texture. Also, along with such a desire, it has become necessary to form the electron beam-passing perforations in high density and minuteness.
However, according to the photoetching method of the prior art generally employed for production of shadow masks, it has been very difficult to form electron beam-passing perforations which are minute and high in precision. More specifically speaking, when minute and highly precise perforations for passing electron beams may be attempted to be formed by use of the technique of the prior art, the resultant perforations for passing electron beams have been those as shown in, for example, FIG. 3(b), wherein the perforations as viewed from the mask face 4 are ununiform in both of positions and shapes, thus being low in precision.
Also, apart from such a problem, as the electron beam-passing perforations are made higher in density and minute, the electron beams shot from electron guns are collided against the shadow mask at increased percentage, whereby the relative positional relation between the perforations and the fluorescent body is changed through the thermal expansion of the shadow mask due to the temperature elevation of the shadow mask to cause a new problem of occurence of color deviation phenomenon called "purity drift" (hereinafter referred to as "PD").